886 
Journal of Agricultural Research 
Vol. XXVIII, No. 9 
would be automatically eliminated. 15 Two comparisons between the density 
of the sap of second-year leaves of 2-2 transplants 16 of western yellow pine and 
that of the smaller roots of the same plants were accordingly made, one in July 
and the other in September. One lot of the roots was thoroughly washed in 
distilled water and air-dried to practically the same condition as that originally 
obtaining on their surface. From another lot as many as possible of the soil 
particles were carefully removed without washing the roots. Both lots were 
packed into test tubes and subjected to the same technique of freezing and press¬ 
ing as the leaves. The results of the tests are given in Table XIV. 
Table XIV .—Comparison of sap densities in leaves and roots 
Depression 
of freezing 
point 
Osmotic 
pressure 
July 27, 1920: 
Leaves....... 
Degrees C. 
1.35 
Atmos¬ 
pheres 
16.2 
Roots (washed in distilled water)...... 
1.17 
14.1 
Roots (unwashed)....... 
1.03 
12.4 
September 20, 1920: 
Leaves... 
1. 48 
17.8 
Roots (washed in distilled water).. .. 
1. 36 
16.4 
These data show that the density of the sap is lower in the roots than it is in 
the tops of the same plants. 17 
It is of interest to note that the density of the cell sap changes during the day, 
increasing as the day advances and decreasing toward evening. The daily 
minimum occurs in the early morning, indicating an apparent relation between 
sap concentration on the one hand and photosynthesis on the other. This 
suggests the observation of Bose (7) who has found that the effects of light and 
warmth are antagonistic. The former induces a retardation and the latter an 
acceleration of growth. In fact, in the later or higher stages of plant succession, 
where competition for light is intense, the plant which is photosynthetically the 
most efficient is most likely to survive because of its greater food supply. 
This is corroborated by the results obtained in the present study. The leaf 
sap of three-year-old seedlings of Douglas fir growing in the Cottonwood Nursery 
under half shade in July gave a depression of the freezing point of 1.39° C., 
denoting an osmotic pressure of 16.7 atmospheres as compared with a freezing 
point depression of 1.59° C. and a corresponding osmotic pressure of 19.1 atmos¬ 
pheres in adjacent unshaded seedlings of the same age and species. In September 
tests on sap of the same two lots of seedlings gave very similar results. The 
shaded seedlings showed a freezing point depression of 1.34° C. and an osmotic 
pressure of 16.1 atmospheres, while the sap of the unshaded seedlings depressed 
the freezing point 1.50° C., indicating an osmotic pressure of 18.0 atmospheres. 
The herbaceous and even some of the ligneous plants growing under a dense 
forest canopy were also found to possess relatively low concentrations. 
There is, moreover, a direct relation between growth form and sap concentra¬ 
tion: The leaves of ligneous plants were found to have a higher osmotic concen- 
14 The recent work of Bouyoucos and his colleagues (8-9,10-18), Hibbard and Harrington (65), and Keen 
(70) is of considerable interest in this connection. 
16 Nursery stock grown two years in the seed bed and two years in the transplant bed. 
17 This conclusion is confirmed by the results of investigations by Hannig (M), Hibbard and Harrington 
(66), and McCool and Millar (90, 91) The latter investigators have also thrown additional light on the 
relation between the concentration of the cell sap and environmental conditions. 
